Fuel efficiency and performance of an internal combustion engine may be improved by varying the timing of intake and/or exhaust valves. One type of variable event valvetrain uses electrically actuated valves to control flow through a cylinder. Electrically actuated valves do not have to be actuated in synchronization with the engine crankshaft. That is, the opening and closing timing of the valves may be varied, within some limits, as desired. One electrically actuated valve design balances an actuator armature between force opposing springs. When the actuator is not powered the balanced spring forces position the armature actuated valve in a neutral position. The neutral position locates the cylinder valve at a partially open position that potentially allows flow into or out of the cylinder. One method to stop an engine cuts power flowing to the valves and stops fuel from flowing to the cylinders upon receiving an engine stop request. As a result, the valves are released to their neutral position and the engine is stopped.
The above-mentioned method can also have several disadvantages. Namely, cutting power to a valve after a request to stop or after a stop can cause gases to enter or exit a cylinder depending on the pressure differential across the valve. This can produce an undesirable popping sound as the valve releases from a closed position and seeks the neutral equilibrium position. Furthermore, by cutting power to the valves little if any control is provided over where or when the cylinder gases exit or enter the cylinder. For example, for an engine having electrically actuated intake and exhaust valves, some cylinders may be at a pressure below atmospheric pressure while others may be at a pressure that is above atmospheric pressure. When the valves are released some of the cylinder contents may be expanded to the exhaust while others may be expanded to the intake manifold. This may increase engine emissions since some cylinder contents may travel through the intake manifold before being vented to atmosphere, for example.
The inventor herein has recognized the above-mentioned disadvantages and has developed a method to control engine valves during stopping and starting that offers substantial improvements.